Pressure reducing valves

1
Pressure reducing valves

• Task

They have the task to provide a constant
secondary pressure p2, independently of the
primary pressure p1 and of the flow rate
Q. Applied for example if a pump delivers fluid
to two different partial systems with two
different pressures. The reduced pressure is
often used for auxiliary functions such as
governing. The pressure reducing valves are
directly operated, pilot operated or three-way
valves.
2
Pressure reducing valve

• Directly operated

In the neutral position the valve is fully
open. If p1 rises and exceeds the spring force
then the valve starts to close. There is a
balance so that the valve opening is exactly as
large that the spring force holds balance with
the wanted p2 pressure. The loss power is
(p1-p2)Q so that it should not operate very long.
Pilot operated similar but we shall not deal
with it.
3
Other pressure control valves

• Pressure difference valves

Has the task to hold the pressure difference
between inlet and outlet independently of the
pressures and the flow rate. Similar to the
pressure reducing valve, except that the spring
side is not connected to the discharge but to one
of the working pressures.
Pressure ratio valves
Have the task to keep the pressure ratio between
inlet and outlet constant, independently of the
pressures and the flow rate. They have no spring,
only a piston with different surface area on both
sides.
Is seldom applied.
4
Flow valves

• Basics

The velocity of hydraulic cylinders can be
influenced by the flow rate. This can be done by
an adjustable pump or by a flow rate control
valve. The flow rate is changed by changing an
orifice surface area or by changing the pressure
difference on a resistance or dividing the flow
rate.

• Advantages of flow valves are
• simplicity
• safe operation
• good dynamic behaviour
• precision

• Disadvantage
• the relatively high energy losses

• Types
• Restrictor valves (throttle valves)
• Flow control valves
• Flow dividing valves

5
Flow valves

• Restrictor valves

These are constant or variable flow rate
resistances. If it is variable then the
restriction area is varied. As far as it is
possible the resistance should be independent of
the Reynolds number. This is easier to do with
orifice-type restrictors. (d gtgt l)
Most common restrictor forms are
6
Flow valves

• Some aspects of a good cross section

• Compact (not sensitive to contamination), best is
  circle
• Progressive growth of surface area, especially
  for small cross-sections (triangle)
• Simple variability of the cross section (all,
  except circle)

Restrictor valves can be

• in series before (bad because of cavitation and
  heat),
• in series after
• parallel to the actuator

Restrictor valves have a bad efficiency. Should
be used only for small powers and short times.
7
Flow valves

• Flow control valves

They consist of a pressure difference valve
combined with a restriction valve. The pressure
difference valve holds the pressure drop constant
over the restriction valve so that the flow rate
depends only on the effective flow area or the
valve displacement x. The movement of the
actuator becomes independent of the load. They
can be either in series or parallel. Parallel
construction has a better efficiency.
a) Series two-way valve
b) Parallel three-way valve
8
Flow valves

• Flow dividing valves

They divide a flow in two parts with a previously
given ratio, independently of the two user
pressures. This ensures a controlled speed of two
actuators independently of the load.
Over the flow restriction 1 and 2 there is a
pressure drop Dp. In equilibrium p1p2, the valve
is in the middle. If p3 is increased, e.g.
because of a higher load, then the flow rate Q1
will decrease. ? p1 increases ? valve is
displaced to the right ? Q1 increases until
p1p2 again. The openings 3 and 4 are amplified
here to a great extent. Other dividing ratios can
be created by asymmetry.
9
Flow valves

• Flow dividing valves

Dynamic behaviour of the valves is
difficult. There are problems at start. There are
errors due to the compressibility of the fluid
and of the pipes. Between flow dividing valve and
user there should be as small fluid volume as
possible and there should not be any valves with
leakage.
Check valves
They have the task to allow the flow in one
direction only. In the other direction the flow
is prevented from flowing usually by a conical or
spherical element (poppet valves). There are also
spool valves but they do not seal so well. The
element is pressed to the seat by a spring
force. Usually needs metallic surfaces with very
accurate machining. Valve seats with elastic
surface did not prove to be very good.
10
Check valves

• Pilot-operated check valves

With an outside pressure the valve can be opened
again. There is a piston with a larger surface
which is operated with a lower pressure than the
closing pressure the valve can be opened. Main
application holding of loads and when desired,
sinking.
11
Check valves

• Sandwich type check valve

Model Z2S
Pilot operated check valve
Main properties Nominal size 625 Maximum
permissible flow 300 l/min System pressure 315
bar Cracking pressure 1,5 3 7,5 10 bar (NG
6 and 10) 2,5 5 7,5 10 bar (NG 16 and 25)
Main properties Nominal size 6150 Maximum
permissible flow 6400 l/min System pressure 315
bar
Typical circuit
12
Check valves

• Applications

This is a simple circuit. If the sinking function
is switched (pilot-operated check valve opens)
then the pressure breaks down in the pumping line
so in the control line too. The check valve
closes again and the procedure starts again. The
pressure builds up and it sinks again. For
continuous sinking there must be some adjustable
restriction valves for braking and ensuring
constant speed.
x
A
B
P
T
13
Check valves

• Applications

Rectifier (Graetz circuit)
Application of the model SV pilot operated check
valve
Application of the model SL pilot operated check
valve, the port A is, e.g. with a throttle-check
valve pre-stressed
Rectifier sandwich plate, model Z4S
14
Big pictures

• End of normal presentation
• Beginning of big pictures

15
Valves

• Directly operated pressure reducing valve

16
Valves

• Directly operated pressure reducing valve

17
Valves

• Directly operated pressure reducing valve

1 - állító elem, 2 - vezérlo vezeték, 3 -
nyomórugó, 4 - vezérlo tolattyú, 5 - vezeto él, 6
- rugótányér résolaj vezeték Y, 7 - visszacsapó
szelep, 8 - méropont
1 adjusting element 2 governing line 3
spring 4 governing spool
5 governing edge 6 spring plate and leakage
line 7 check valve 8 measuring point
18
Flow control valves

• Two-way valve

19
Flow control valves

• Two-way valve

20
Flow control valves

• Two-way valve

21
Flow control valves

• Three-way valve

22
Flow control valves

• Three-way valve

23
Flow control valves

• Three-way valve

24
Valves

• Check valve

25
Valves

• Check valve

26
Valves

• Pilot-operated check valves,
• Internally drained, without decompression poppet
  option

1 - main poppet 3 - spring 4 - piston X - pilot
port Y drain port
27
Valves

• Pilot-operated check valves,
• Internally drained, with decompression poppet
  option

pSt pilot pressure p1 pressure on the port
B p2 pressure on the port A A1 surface of the
main poppet A2 surface of the decompression
poppet A3 surface of the pilot piston AK
piston surface at the cylinder AR ring surface
at the cylinder F load at the piston FF
spring force with friction
1 main poppet 2 decompression poppet 3 -
spring 4 - piston X operating path
larger picture
28
Valves

• Pilot-operated check valves,
• Externally drained, with decompression poppet
  option

29
Valves

• Pilot-operated check valves

30
Valves

• Sandwich type check valve

31
Valves

• Sandwich type check valve

32
Valves

• Sandwich type check valve

33
Valves

• Sandwich type check valve

34
Valves

• Rectifier
• (Graetz circuit)

35
Valves

• Rectifier
• sandwich plate,
• model Z4S

36
Valves

• Application of pilot operated check valve

37
Valves

• Application of pilot operated check valve

38
Valves

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